FTS is an integral part of the Gas-To-Liquids, Coal-To-Liquids and Biomass-To-Liquids processes in which natural gas, coal or biomass respectively is converted, by means of three main process steps, into liquid transportation fuels and/or chemicals. The three steps are (i) natural gas, coal or biomass conversion into synthesis gas, i.e. a mixture of carbon monoxide and hydrogen, (ii) FTS which converts the synthesis gas mainly into hydrocarbon compounds often including waxy organic compounds, and (iii) processing of the hydrocarbon compounds to produce liquid transportation fuels and/or chemicals, especially by hydroprocessing of the waxy organic compounds. For a slurry phase FTS reaction to produce waxy organic compounds, a cobalt based catalyst is immersed in the synthesized waxy compounds inside a three phase slurry bed reactor.
When cobalt-based Fischer-Tropsch synthesis catalysts are used in FTS, including slurry phase Fischer-Tropsch synthesis, they partially lose activity over time, so that the catalysts thus become spent. Spent cobalt-based Fischer-Tropsch synthesis catalysts can be regenerated which will recover, to a large extent, the FTS performance of the spent catalyst. Regeneration can be executed outside the FTS process by sequentially subjecting the spent catalyst to dewaxing, oxidation and reduction.
GB 2 222 531 dealing with the regeneration of a fixed bed Fischer-Tropsch catalyst that was used for the preparation of hydrocarbons by catalytic reaction of carbon monoxide with hydrogen, acknowledges the fact that temperature control is important during regeneration of a spent catalyst with oxygen. Its solution to this issue is (i) to remove the excess hydrocarbons by means of a solvent wash step prior to the oxidation of the catalyst, and (ii) to perform the oxidation at low oxygen concentrations, i.e. 0.1-3 vol %. The drawback of using low oxygen concentrations is the extended cycle time of the oxidation process. By cycle time is meant the length of time it takes to complete the oxidation process.
WO 02/085508 deals with the regeneration of cobalt based Fischer-Tropsch catalysts that were used in slurry phase Fischer-Tropsch synthesis processes. The spent catalyst is removed from the Fischer-Tropsch reactor; de-waxed and dried to produce a free flowing powder; oxidized with oxygen, typically in a fluidized bed, to remove hydrocarbons and carbonaceous materials; the catalyst reduced; and the catalyst powder re-slurried to obtain a regenerated slurry catalyst. The oxygen treatment has the following objectives: (i) removing the hydrocarbons and carbonaceous materials, and (ii) oxidizing the reduced cobalt to cobalt oxide. Additionally, unwanted temperature excursions due to the exothermic nature of the above-mentioned objectives, must be minimized. Minimizing the exothermic reactions is done by performing the oxidation at low oxygen concentrations and low heating rates. The drawback of using low oxygen concentrations and low heating rates is the extended cycle time of the oxidation process.
Spent cobalt Fischer-Tropsch catalysts can thus be regenerated. To speed up the oxidation step, as part of the regeneration process, and to decrease the cycle time, high oxygen concentrations should be used. It is, however, difficult to control the catalyst temperature during the oxidation, and the catalyst temperature can rise to unacceptably high levels, resulting in failure to recover catalyst activity sufficiently and damage to the regeneration equipment. The uncontrolled temperature rise also poses a safety risk in a commercial regeneration facility. This is even more important when high oxygen concentrations are used.
It is thus an aim of this invention to provide a process for regenerating a spent cobalt-based Fischer-Tropsch synthesis catalyst whereby this problem is at least alleviated.
As indicated above, it is important to be able to control the exothermic reaction during the oxidation treatment. The Applicant is aware that the catalyst temperature during the oxidation treatment can be controlled by using diluted oxygen, e.g. 3 vol % oxygen in nitrogen, thereby limiting the rate at which heat is generated. However, when moving to higher oxygen concentrations during the oxidation treatment, which is desirable in order to reduce the time taken for the oxidation treatment, it was found that it is difficult to control the catalyst bed temperature. Thus, for example, when the oxidation treatment is performed at oxygen levels representative of air, i.e. around 21 vol %, it was found that satisfactory control of the catalyst bed temperature was not achieved.